U.S. patent application number 09/808299 was filed with the patent office on 2002-11-14 for vortex-induced vibration reduction device for fluid immersed cylinders.
Invention is credited to Measamer, Jeffery C., Qi, Tao, Xu, Qi.
Application Number | 20020168232 09/808299 |
Document ID | / |
Family ID | 25198397 |
Filed Date | 2002-11-14 |
United States Patent
Application |
20020168232 |
Kind Code |
A1 |
Xu, Qi ; et al. |
November 14, 2002 |
Vortex-induced vibration reduction device for fluid immersed
cylinders
Abstract
A mechanism to be applied to an exterior surface of a
cylindrical structure for reduction of the effect of Vortex Induce
Vibration (VIV) in the cylindrical structure when immersed in
flowing fluid. The mechanism is provided with a generally
cylindrical column having a central axis, an interior surface
corresponding in size and shape to the exterior surface of the
cylindrical structure to which the mechanism is to be applied and
an outer surface defining a wall thickness. A reduced wall
thickness is formed into the outer surface in a pattern to produce
a discontinuity that interrupts the lengthwise coherence of vortex
shedding of moving fluid from the outer surface when the
cylindrical column is attached to the exterior of the cylindrical
structure in the flowing fluid. The effect of VIV on the
cylindrical structure is effectively reduced. A submergible
cylindrical assembly for positioning in a flowing body of water and
having enhanced resistance to vortex induced vibration is
disclosed. The cylindrical assembly comprises a cylinder having an
axis, an outer surface and a wall thickness. The cylinder has a
pattern cut into the outer surface thereof that selectively reduces
the wall thickness of the cylinder such that the formation of
vortices is reduced, thereby reducing or eliminating the lift force
on the cylinder and reducing or eliminating the vortex induced
vibration that may weaken or damage the cylinder.
Inventors: |
Xu, Qi; (Katy, TX) ;
Qi, Tao; (Katy, TX) ; Measamer, Jeffery C.;
(Houston, TX) |
Correspondence
Address: |
Theodore F. Shiells
Gardere Wynne Sewell LLP
3000 Thanksgiving Tower
1601 Elm Street, Suite 3000
Dallas
TX
75201-4767
US
|
Family ID: |
25198397 |
Appl. No.: |
09/808299 |
Filed: |
March 14, 2001 |
Current U.S.
Class: |
405/224 ;
166/367 |
Current CPC
Class: |
F16L 1/123 20130101;
E21B 17/01 20130101 |
Class at
Publication: |
405/224 ;
166/367 |
International
Class: |
E21B 017/01 |
Claims
What is claimed is:
1. A mechanism to be applied to an exterior surface of a
cylindrical structure for reduction of the effect of Vortex Induce
Vibration (VIV) in said cylindrical structure when immersed in
flowing fluid, the mechanism comprising: a generally cylindrical
column having a central axis, an interior surface corresponding in
size and shape to said exterior surface of said cylindrical
structure to which said mechanism is to be applied, an outer
surface and a wall thickness; and a reduced wall thickness formed
into said outer surface in a pattern to produce a discontinuity
that interrupts the lengthwise coherence of vortex shedding from
said outer surface when said cylindrical column is attached to the
exterior of said cylindrical structure in said flowing fluid,
thereby reducing the effect of VIV on said cylindrical
structure.
2. The mechanism for reduction of VIV as recited in claim 1 wherein
the cylindrical structure further comprises a hull of an offshore
vessel.
3. The mechanism for reduction of VIV as recited in claim 1 wherein
the cylindrical structure further comprises a drilling riser.
4. The mechanism for reduction of VIV as recited in claim 1 wherein
the cylindrical structure further comprises a production riser.
5. The mechanism for reduction of VIV as recited in claim 1 wherein
the cylindrical structure further comprises a hybrid riser.
6. The mechanism for reduction of VIV as recited in claim 1 wherein
the generally cylindrical column further comprises a composite
material on said outer surface..
7. The mechanism for reduction of VIV as recited in claim 1 wherein
said reduced wall thickness formed in said pattern further
comprises a plurality of notches.
8. The mechanism for reduction of VIV as recited in claim 7 wherein
said plurality of notches further comprise notches having a right
angled triangular cross-sectional shape.
9. The mechanism for reduction of VIV as recited in claim 7 wherein
said plurality of notches further comprise notches having a
rectangular cross-sectional shape.
10. The mechanism for reduction of VIV as recited in claim 7
wherein said pattern comprises said plurality of notches formed at
a plurality of different circumferential positions along a length
of said cylindrical column.
11. The mechanism for reduction of VIV as recited in claim 10
wherein an angular change in said different circumferential
positions for a predetermined interval of length is between about
10 and 90 degrees for a preselected interval of length equal to
between about 0.5 times the diameter and 10 times the diameter.
12. The mechanism for reduction of VIV as recited in claim 11
wherein said angular change in circumferential positions for each
preselected interval of length is approximately 30 degrees.
13. The mechanism for reduction of VIV as recited in claim 7
wherein the plurality of notches are elongated in the axial
direction of the cylindrical column.
14. The mechanism for reduction of VIV as recited in claim 13
wherein the plurality of notches elongated in the axial direction
of the cylindrical column are substantially parallel to the axis of
said cylindrical column.
15. The mechanism for reduction of VIV as recited in claim 13
wherein the plurality of notches elongated in the axial direction
of the cylindrical column are at an angle relative to the axis of
said cylindrical column.
16. The mechanism for reduction of VIV as recited in claim 1
wherein said pattern of reduced wall thickness further comprises a
helical pattern.
17. The mechanism for reduction of VIV as recited in claim 1
wherein said generally cylindrical column further comprises a
plurality of axially aligned generally cylindrical shaped columnar
sections and wherein said reduced wall thickness formed in a
pattern along the length of said generally cylindrical column
comprises at least one notch formed into an outer surface on each
of said plurality of axially aligned columnar sections and said at
least one notch on each columnar section at a different
circumferential position relative to each adjacent columnar
section.
18. The mechanism for reduction of VIV as recited in claim 17
wherein said generally cylindrical shaped columnar sections further
comprise a plurality of sets of segments of a cylinder and a
plurality cylindrical shape band clamps for holding said segments
onto said exterior surface of said cylindrical structure to form
said cylindrical columnar sections at least one of each set of said
segments having a notch formed into an outer surface thereof and
each set at different circumferential positions relative to each
other along a length of said cylindrical structure to place said
notches in said pattern for interrupting said lengthwise coherence
of vortex shedding.
19. The mechanism for reduction of VIV as recited in claim 18
wherein said segments of each cylinder forming set comprise
junction ends having the same wall thickness as the junction ends
of each adjoining segment to form a smooth cylindrical surface at
said junction and said notch is formed in said at least one segment
spaced form either junction end.
20. The mechanism for reduction of VIV as recited in claim 19
wherein said segments of each cylinder forming set comprise
junction ends having the different wall thickness at the junction
ends of each adjoining segment to form a reduced wall thickness and
discontinuity at said junction.
21. A method for reducing vortex induced vibration in a generally
vertically submerged cylinder comprising the steps of: cutting a
pattern of reduced wall thickness areas in the outer surface of a
cylinder to be submerged; and submerging said cylinder with said
pattern cut into said outer surface such that the lengthwise
coherence of vortex shedding is interrupted and the lift forces on
the cylinder from said vortex shedding at one position along the
length of the cylindrical structure are out of phase and cancel
lift forces at another position, thereby reducing the effect of
vortex induced vibration on the cylinder.
22. The method as recited in claim 21 wherein the step of cutting a
pattern in the outer surface of the cylinder further comprises
cutting a pattern in the outer surface of a hull of an offshore
vessel.
23. The method as recited in claim 21 wherein the step of cutting a
pattern in the outer surface of the cylinder further comprises
cutting a pattern in the outer surface of a drilling riser.
24. The method as recited in claim 21 wherein the step of cutting a
pattern in the outer surface of the cylinder further comprises
cutting a pattern in the outer surface of a production riser.
25. The method as recited in claim 21 wherein the step of cutting a
pattern in the outer surface of the cylinder further comprises
cutting a pattern in the outer surface of a hybrid riser.
26. The method as recited in claim 21 wherein the step of cutting a
pattern in the outer surface of the cylinder further comprises
cutting a pattern in the outer surface of a composite material on
the outer surface of a riser.
27. The method as recited in claim 21 wherein the step of cutting a
pattern in the outer surface of the cylinder further comprises
cutting a plurality of notches in the cylinder.
28. The method as recited in claim 27 wherein the step of cutting a
plurality of notches further comprises cutting a plurality of
notches having a right angled triangular cross-sectional shape.
29. The method as recited in claim 27 wherein the step of cutting a
plurality of notches further comprises cutting a plurality of
notches having a rectangular shaped cross-section.
30. The method as recited in claim 27 wherein the step of cutting a
plurality of notches in the cylinder further comprises periodically
changing the angular location of the notches on the cylinder at a
constant interval of length along the cylinder.
31. The method as recited in claim 30 wherein the step of
periodically changing the angular location of the notches on the
cylinder at a constant interval further comprises periodically
changing the angular location of the notches on the cylinder
between 10 and 90 degrees for each interval of length equal to
between 0.5 times the diameter and 10 times the diameter.
32. The method as recited in claim 30 wherein the step of
periodically changing the angular location of the notches on the
cylinder at a constant interval further comprises periodically
changing the angular location of the notches on the cylinder
approximately 30 degrees for each interval of length equal to about
1.5 times the diameter.
36. The method as recited in claim 27 wherein the step of cutting a
plurality of notches further comprises cutting a plurality that are
parallel to the axis of the cylinder.
37. The method as recited in claim 15 wherein the step of cutting a
pattern in the outer surface of the cylinder further comprises
cutting a helical pattern.
38. A submergible cylindrical assembly for positioning in a flowing
body of water and to reduce the effect of vortex induced vibration
(VIV), the cylindrical assembly comprising: a cylindrical column
having an axis, an outer surface, a wall thickness and a length;
and a pattern cut into the outer surface of said cylindrical column
reducing the wall thickness at preselected locations, the pattern
comprising a plurality of notches positioned such that the angular
location of the notches on the outer surface of the cylindrical
column changes along the length of the cylindrical column, thereby
interrupting the lengthwise coherence of vortex shedding which
reduces the effect of VIV on the cylindrical assembly.
39 The cylindrical assembly as recited in claim 38 wherein the
cylindrical column further comprises a spar of an offshore
vessel.
40. The cylindrical assembly as recited in claim 38 wherein the
cylinder further comprises a drilling riser.
41. The cylindrical assembly as recited in claim 38 wherein the
cylinder further comprises a production riser.
42. The cylindrical assembly as recited in claim 38 wherein the
cylinder further comprises a hybrid riser.
43. The cylindrical assembly as recited in claim 38 wherein the
cylinder further comprises a composite material on the outer
surface.
44. The cylindrical assembly as recited in claim 39 wherein the
notches further comprise right angled triangular notches.
45. The cylindrical assembly as recited in claim 38 wherein the
interval is approximately 30 degrees.
46. The cylindrical assembly as recited in claim 38 wherein the
direction of the notches is parallel to the axis of the
cylinder.
47. The cylindrical assembly as recited in claim 38 wherein the
pattern further comprises a helical pattern.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] This invention relates in general to the field of solid and
hollow cylinders, such as risers, hoses, pilings and pipes immersed
in a fluid subject to relative motion between the cylinder and the
fluid. In particular, the invention relates to a device and
mechanism and method for reducing vortex-induced vibration caused
by relative movement of water past a cylinder and also to a
cylindrical assembly incorporating the inventive mechanism.
BACKGROUND OF THE INVENTION
[0002] Without limiting the scope of the invention, its background
will be described primarily with reference to offshore risers used
in sub-sea production wells as an example. Submerged
cylindrically-shaped objects, such as risers, spars, or other
elongated cylindrical structures used for under-sea oil or gas
production, pumping, or loading are often exposed to relative
movement of a body of fluid, particularly moving sea currents. Such
elongated cylindrical structures are common in offshore petroleum
exploration, production and transportation. Sometimes such
elongated cylindrical structures extend from the surface to
hundreds of meters below the surface, as in the case of spar
platforms for production. Sometimes the cylindrical structures
extend from the seabed thousands of meters upward toward the
surface and into sea currents, as in offshore production risers,
loading and unloading risers or hybrid risers for petrochemical
production or transport. Cylindrical riser structures may support
on their exterior or encase one or more pipelines or risers
extending from the seabed to a drilling or production platform, to
a ship or to another offshore structure or vehicle. Such risers or
cylindrical riser support structures are continuously exposed to
ocean currents that produce vortexes or vortices that tend to
travel downstream with the current as the water moves around and
past the risers. These vortices produce oscillating "lift" forces
on the cylindrical structure as a result of vortex shedding and the
spanwise, or lengthwise, coherence of the vortex shedding can
produce substantial cumulative lift force on the elongated
cylindrical structure. The effect is particularly adverse in the
case of a cylindrical riser support column extending several
hundreds of meters in the path of the current.
[0003] The lift forces due to vortex shedding act generally normal
to the axis of the cylindrical structure and flow direction. As a
vortex is produced and then separated in a "sheet" from the
cylindrical surface along the length or span of the cylinder
exposed to the current, the lift force can be significant and
destructive. The vortices are swirling currents that repeatedly
shed from the cylinder, sometimes called "Von Karman Vortex Sheets"
and produce vortex-induced vibration. The vibratory movement or
vortex-induced vibration (VIV) Von Karman Vortex caused by the
repeated sheet separation from the cylinder is sometimes called
"Aeolian Vibration." This vortex-induced vibration creates cyclic
stresses on the cylindrical structure that may be too small to
cause immediate fracture, but upon constant repetition may weaken
or damage the riser through material fatigue or stress-induced
fracture. In certain relatively common current situations, a
resonant vibration can be created, causing repetitive forces in
phase with the vibratory motion that can overstress the cylindrical
structure to potentially catastrophic failure.
[0004] In the past, fins protruding from the peripheral surface of
the cylinders exposed to the current or other fluid movement, as in
production riser situations, were used to reduce the adverse effect
of such vortex formation and vortex sheet shedding. For example,
helically-arranged vortex-shedding ribs, or strakes, have been
designed to be installed on submerged risers exposed to ocean
currents. In one prior device, such strakes are to be incorporated
as components of a flexible wrap or panel to be disposed about and
secured to the submerged riser. Typically the strakes are to be
clamped to the riser prior to its being submerged. Such strakes
could be formed by pairs of clamping flanges mounted along the
adjacent edges of elongated parallelogram-shaped wrap segments. The
wrap segments could be positioned side-by-side, twisting around the
outer surface of the riser, and then bolted to engage at the clamp
flanges, forming a helical strake extending in a spiral around and
along the length of the cylindrical structure that will be exposed
to moving current.
[0005] In another design, one or more ribs or strakes could be
attached vertically or diagonally on a flat, rectangular panel of
flexible wrapping material. The wrapping material would be
dimensioned to encircle, by itself, an elongated segment of a
single riser, piling, pipe or other cylindrical object. Clamping
flanges were to be mounted along opposed vertical edges of the
rectangular panel. The clamping flanges were to be brought together
and clamped, thereby stretching the panel to wrap securely around
and frictionally embrace the outer surface of the riser. A
plurality of such wrapped panels with ribs or strakes were to be
clamped in deployed positions, along the length of the cylindrical
structure such that the strakes were aligned at either end of
adjacent panels in a helical configuration encircling the wrapped
riser structure.
[0006] It is difficult to transport, handle and install a
cylindrical riser support structure having protruding strakes.
Further, it has been found that installation underwater at the
riser site is extremely difficult and usually impractical. It has
been found that fabrication of a cylindrical riser structure with a
protruding strake of a prior design is costly. Additionally, it has
been found that the protruding strake on a cylindrical riser
support structure increases the viscous drag of the water against
the riser assembly, thereby risking greater stress and requiring
increased size and strength for the riser support design.
[0007] In certain riser installations, a polymeric coating and, in
particular, a polymeric foam layer is applied to the exterior
surface of the risers and the riser support cylinder to provide
protection from the undersea environment and advantageously to
provide buoyancy to the assembly. The riser itself may be composed
of a metal or a composite material. The riser support structure is
normally a metal support cylinder with the metal or composite
cylindrical riser pipe lines and polymeric foam coating material
attached to the surface of the metal cylinder to facilitate
maintaining the riser and support structure in an upright position
by reducing the combined mass density (i.e., by adding buoyancy).
It has been found that securing strakes, of any prior known design,
to the exterior of a layer of polymeric foam is difficult. For
example, clamping of strakes to the polymeric surface often fails
due to insufficient compression strength of the foam. Particularly,
in the case of a polymeric foam coating or bundle on the riser or
riser support cylinder, clamping tension may not be sufficient to
maintain the strakes in a secure position. Excessive clamping
tension can significantly reduce the buoyancy by crushing the foam
layer.
[0008] A need has therefore arisen for a device, mechanism and
method to reduce, resist or suppress vortex induced vibration
(VIV), or the effect of VIV on submergible cylinders such as risers
and riser support columns, without requiring the attachment of a
protruding strake. A need has also arisen for a submergible riser
assembly with a VIV reduction mechanism attached that is easy to
transport, easy to handle and easy to install and that is not
costly to fabricate. In addition, a need has arisen for such a VIV
reduction mechanism for fluid immerse cylindrical structures and
assemblies, including submergible riser assemblies that does not
significantly increase the viscous drag of moving fluid or moving
water against the immersed cylinder or submerged riser
assembly.
SUMMARY OF THE INVENTION
[0009] The present invention disclosed herein comprises a device,
mechanism and method for use in a generally cylindrical assembly
that is resistant to vortex-induced vibration when immersed in a
moving fluid. The generally cylindrical assembly of the present
invention, and particularly in the case of a cylindrical riser
assembly, is easy to transport, handle and install and is not
costly to fabricate. In addition, a feature of one embodiment of a
cylindrical assembly according to certain inventive aspects of the
present invention is that the cylindrical assembly is submergible
in a body of water and resists or reduces vortex-induced vibration
(VIV) and does not significantly increase the viscous drag of the
fluid or water moving past the cylindrical assembly.
[0010] The vortex induced vibration (VIV) reduction mechanism of
the present invention and the submergible cylindrical assembly of
the present invention having such VIV reduction mechanism combined
therewith effectively reduce the adverse effect of vortex-induced
vibration when positioned in a flowing body of fluid such as water.
The VIV reduction mechanism comprises a generally cylindrical
column having a central axis, an outer surface, a wall thickness
and a length. A pattern is cut or formed into the outer surface of
the generally cylindrical column to selectively decrease the
distance of the outer surface from the central axis. The pattern
may be formed with a plurality of columnar sections each having a
notch cut into the outer surface. A plurality of columnar sections
are placed in series or stacked along the length of the cylindrical
column. The notch of each columnar section is positioned in a
selected circumferential angular relationship with the notch of
each other columnar section and extends partially along the length
of the column, thereby selectively reducing the thickness of the
wall and producing a discontinuity in the outer cylindrical surface
at selected positions. The angular position of each notch or of
each reduced thickness portion of a wall around the circumference
of the generally cylindrical column sections is differently
selected along the length of the column. The selected angular
positions provide a pattern of discontinuities on the generally
cylindrical outer surface of the column. It will be understood that
for a solid cylinder the wall thickness is nominally equal to the
nominal radius. For a riser support column comprising a hollow
cylinder encased in a polymeric or foam material, the wall
thickness is less than the nominal radius. Selectively decreasing
the distance from the axis to the surface might also be considered
the same as reducing the wall thickness at selected locations or in
a desired pattern. The reduced radius or reduced wall thickness
preferably provides a sharp discontinuity in the surface.
[0011] Preferably, the discontinuities will be selectively and
appropriately positioned in a pattern, desirably a helical pattern,
along the length of the column so that the VIV effect of vortex
sheet separation from the cylindrical column is reduced. Forming or
approximating a helical shaped discontinuity along the length of
the cylindrical structure exposed to moving current facilitates
reduction of VIV, or at least reduces its negative effects in the
cylindrical structure. The discontinuity acts to shed the vortex at
different times at different segments along the length of the
cylinder. The various vortex-created lift forces are out of phase
from each other and thus are out of phase with the oscillation that
the forces would otherwise cause in the cylindrical structure at
any given time. The "out of phase" forces tend to cancel each other
out. Thus, the vibratory effect of vortex-induced lift forces on
the cylinder are reduced.
[0012] The abrupt reduction in thickness or the formation of a
sharp discontinuity in the outer surface is generally accomplished
using variously shaped notches or grooves. Preferably, notches or
grooves having sharp corners have been found to be useful, such as
a right angle triangular-shaped notch, an equilateral
triangular-shaped notch, a rectangular-shaped notch, or other
angular polygon. The notches or reduced thickness areas causing
discontinuities in the outer surface of the cylindrical structure
are either formed in a substantially continuous helical pattern or
formed with segments of notches that are rotated to different
angular positions at regular intervals along the length of the
cylindrical structure. By forming relatively short segments of
longitudinal notches and sequentially rotating each notch
consistent angular amounts (between 10.degree. and 90.degree.) at
regular intervals of length (between about 0.1 and 10 times the
diameter), a long helical shape is approximated by the plurality of
rotated notches or grooves. A series of partially rotated column
sections, each column section having vertical or slightly angled
notches or grooves may be provided along the length of the
cylindrical column structure. By rotating the column sections at
the time they are affixed to the support cylinder, a helically
shaped groove is approximated by vertically elongated notches. A
better approximation of a helical groove may be formed by a series
of columnar sections having angled notch segments aligned end to
end by rotating the columnar sections.
[0013] A generally cylindrical column structure to which the
present inventive VIV reduction mechanism is to be applied
according to the disclosure herein, might typically be a support
structure for drilling risers or production risers. It will be
understood that this is by way of example only of the cylindrical
structure to which the VIV reduction device and mechanism is
applied. The resulting inventive VIV reduced cylindrical assembly
may also be used for other cylindrical structures; i.e., it may be
a drilling riser, a production riser, a hybrid riser and/or any
number of other elongated cylindrical structures that may be
subjected to the adverse effects of VIV. The cylindrical structure
may comprise a solid metal outer surface or may comprise a
composite material on which a VIV reduction mechanism is secured or
formed. The VIV reduction mechanism may be notches or grooves
formed in the solid surface. Preferably notches or grooves in
helical pattern may be formed into a composite polymeric material
or a polymer foam material secured, attached or formed onto the
surface of a generally cylindrical support structure such as a
riser support cylinder or cut or molded into the surface of a
generally cylindrically shaped polymer foam material attached on
the outer surface of any immersed cylindrical structure. The VIV
reduction mechanism may also be formed in a composite structure
with notches, grooves or other discontinuity formed into the outer
surface or into the wall thickness, as described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The foregoing objects, advantages, and features, as well as
other objects and advantages, will become apparent with reference
to the description, claims and drawings below, in which like
numerals represent like elements and in which:
[0015] FIG. 1 is a schematic perspective, partially cutaway view,
depicting various undersea uses of cylindrical columns in moving
fluid (i.e., vertical cylindrical columns in horizontal water
currents).
[0016] FIG. 2 is a schematic depiction of a cylindrical riser
bundle support assembly provided with an upper buoyancy can, and a
cylindrical support structure for the bundle of tubular risers with
the cylindrical support structure having applicants' VIV
suppression invention applied to the cylindrical exterior
surface;
[0017] FIG. 3 is a schematic depiction of a hybrid riser assembly
having a cylindrical riser support structure with a portion thereof
having, for additional buoyancy, substantially cylindrical foam to
which applicants' inventive VIV suppression device has been
applied;
[0018] FIG. 4 is a schematic depiction of a representative segment
of the upper enhanced buoyancy portion of the substantially
cylindrical riser structure of FIG. 3 in which a plurality of
risers are held together supported by a central support cylinder in
segmented foam quadrants clamped in a substantially cylindrical
shape and having segments of applicants' VIV reduction devices
applied and clamped to the exterior of the enhanced buoyancy foam
riser bundle;
[0019] FIG. 5 is a schematic cross-sectional depiction of one
embodiment of applicants' inventive VIV reduction device and
mechanism in which four sections of the VIV suppression device are
depicted for clamping around a riser, two of which in each
cylindrical segment have a notch or sharp discontinuity formed
therein with each notch at concentric opposed locations, the
junctions at each end each section being concentric with the other
ends and of the same width so that clamping engagement results in a
smooth transition between one half and the other;
[0020] FIG. 6 shows an embodiment of the VIV suppression device in
which four discontinuities or four notches or four "step notches"
are formed in four quadrants of the VIV columnar segments;
[0021] FIG. 7 shows an embodiment similar to FIG. 6, except that
each VIV reduction columnar segment is divided into two
substantially identical pieces. The cut can be anywhere in the
segment;
[0022] FIG. 8 shows another embodiment similar to FIG. 7, except
that each VIV reduction columnar segment is divided into four
identical pieces which lock each other together. This embodiment
will allow the load on the notches to be better distributed along
the entire length of the segment.
[0023] FIG. 9 shows the arrangement of the segments and notches
depicted in FIGS. 6-8 in the longitudinal direction. For clarity
only one notch on each columnar segment is shown.
[0024] FIGS. 10-13 show cross-sections of the segments of FIG. 9
taken along the lines 10-10, 11-11, 12-12, and 13-13,
respectively.
[0025] FIG. 14 shows another arrangement of the notches depicted in
FIGS. 6-8 in the longitudinal direction. In this embodiment,
successive notches form a spiral line. For clarity, only one notch
on each columnar segment is shown.
[0026] FIGS. 15-18 show cross-sections of the segments of FIG. 14
taken along the lines 15-15, 16-16, 17-17 and 18-18,
respectively.
[0027] FIG. 19 shows another embodiment in which the outline of the
columnar segment is not a circle, with the phantom line in the
drawing showing a circle (that is not part of a structure) for
comparison. At one side, the surface extends beyond the circular
phantom line and at the other side it is inside the circular
phantom line. The notch arrangement of successive segments in the
longitudinal direction can be the same as depicted in FIGS. 9 and
14.
[0028] FIG. 20 shows another embodiment similar to FIG. 19, except
that the columnar VIV reduction segment is divided into two
identical pieces. The notch arrangement of successive segments in
the longitudinal direction can be the same as depicted in FIGS. 9
and 14.
[0029] FIG. 21 shows another embodiment of a segment that has a
notch of a different shape. The notch arrangement in the
longitudinal direction can be the same as depicted in FIGS. 9 and
14.
[0030] FIG. 22 is a side view of longitudinally arranged segments
with triangular notches. The triangular notches cover entire
cylindrical surface and in the longitudinal direction, the notches
forming spiral (helical) lines.
[0031] FIG. 23 is a cross-sectional view of one of the segments of
FIG. 22, taken along the line 23-23.
[0032] FIGS. 24 and 25 show another embodiment where the cross
section of the segment is an ellipse and the angular orientation of
the long axis of each rotates, as shown in the cross-section in
FIG. 25, to form a spiral (twisted) shape.
[0033] FIGS. 26 and 27 show another embodiment where the cross
section is a triangle with rounded corners. The angular orientation
of each triangle rotates, as shown in the cross-section in FIG. 27,
to form a spiral (twisted) shape.
[0034] FIGS. 28 and 29 show another embodiment where the cross
section is a square with rounded corners. The angular orientation
of the square rotates, as shown in FIG. 29, to form a spiral
(twisted) shape.
[0035] FIGS. 30 and 31 show another embodiment where the cross
section is an ellipse. The angular orientation of the long axis of
the ellipse rotates, as shown in FIG. 31, to form a discontinuous
stepped pattern.
[0036] FIGS. 32 and 33 show another embodiment where the cross
section is a triangle with rounded corners. The angular orientation
of the triangle rotates, as shown in FIG. 33, to form a
discontinuous stepped pattern.
[0037] FIGS. 34 and 35 show another embodiment where the cross
section is a square with rounded corners. The angular orientation
of the square rotates, as shown in FIG. 35, to form a discontinuous
stepped pattern.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] While the making and using of various embodiments of the
present invention are discussed in detail below, it should be
appreciated that the present invention provides many applications
for the inventive concepts which can be embodied in a wide variety
of specific contexts. The specific embodiments discussed herein are
merely illustrative of specific ways to make and use the invention
and do not delimit the scope of the invention.
[0039] Referring to FIG. 1, which is a schematic depiction of
floating production systems on the sea surface 10 and extending
from the seabed 12 through a distance of ocean, including a portion
14 having sea currents and a portion 15 without significant sea
currents. Examples of various ocean equipment to which the
invention may be usefully applied are depicted, including a sea
floor drilling rig 16, a ship 18, a columnar-supported drilling
platform 20, a floating production platform 22 and a spar platform
24, as well as a collection vessel 26. Risers 28 are shown
extending from the seabed 12 to the collection ship 18 where
hydrocarbons are pumped on board from the risers and transported to
an appropriate port facility where similar risers may offload the
petroleum products to a refinery. The drilling or production
platform 20 is schematically depicted with a drill casing 30
extending to the floor surface and also support legs 32 on which
the drilling or production platform is secured to the sea floor
12.
[0040] The spar platform hull 24 is supported on a large
cylindrical spar hull 40 having a heavy end 39 and an upwardly
buoyant end 37 so that the platform 24 is floating in a desired
position and may be anchored in position with mooring lines 41. Top
tension risers and steel catenary riser pipes 42 extend upward to
the spar platform 24 and through or about the spar hull 40 to the
production platform 24. The collection vessel 26 is shown receiving
hydrocarbon from a hydrocarbon collection system 44 for sub-sea
wells on the seabed 12 and providing the produced hydrocarbons
through upwardly extending risers 46 and also collecting
hydrocarbons from the well 16 through elongated recovery pipes 48
that may extend flexibly along the seabed 12 and upward to
collection vessel 26.
[0041] The foregoing floating production systems are depicted by
way of background so that uses of the inventive VIV reduction
mechanism according to various embodiments of the present invention
may be more fully understood as to the wide ranging applications to
riser cylinders drill casings, riser support columns, pipes,
platform legs, cylindrical spars and other similar immersed
cylindrical structures.
[0042] With reference to FIG. 2, a production/transport vessel 50,
in this case a ship 50, is shown in position for receiving
hydrocarbons above a buoyancy canister 52 attached to a riser
support cylinder 54 so that the riser support cylinder 54 may be
held upright and having a connection in 53 held adjacent to the sea
surface 10. Depicted in FIG. 2 is one embodiment of VIV reduction
mechanism 56 attached along a length 14 exposed to current 58 that
is depicted as horizontal arrows 58. In shallow waters, the current
58 may extend from the sea surface 10 to the seabed 12, however, in
deep waters as is often the case, the current 58 may extend a
length 14 that may be several hundred to several thousand meters
deep. In situations where the sea depth is thousands of meters,
there will also be a length 15 of riser 54 that is not exposed to
any significant current. In situations where no VIV reduction
mechanism 56 is applied to the cylindrical riser support, the
current 58 will form vortexes or a sheet of vortex material along
substantially the entire length 14 exposed to the current 58. With
vortex reduction mechanisms 56 applied to riser support structure
54, the vortices 60a, b, c, d, e, f, and g will each shed from the
column surface at different times and/or different locations such
that the lifting force at each longitudinal position along the
riser support structures is out of phase with the oscillation of
the entire riser 54 thereby canceling out the vibration. This
effectively reduces the vibration.
[0043] The vessel 50 is shown held in place with anchor cable 62
attached to sea anchors 64 so that the conduits 66 from the
connection head 53 to the production vessel 50 are retained in a
relatively stable position. The VIV reduction mechanism 56 applied
along cylindrical riser 54 comprises a plurality of VIV reduction
column segment 70. These have been labeled starting at the topmost
as VIV reduction column segment 70a with the next columnar segment
70b, 70c and etc. Each columnar segment is rotated relative to the
next such that a sharp notches, grooves or discontinuities 72a, b,
c, d, e, etc. are provided in each columnar segment.
[0044] Advantageously, the discontinuity areas are rotated
angularly with each successive columnar segment to a different
angular position relative to the adjacent columnar segments.
Desirably, for example, segment 70b is rotated an angle of between
about 10.degree. and 90.degree. relative to segment 70a. Also
desirably segment 70c is also rotated to the same angular amount
relative to 70b as 70b is rotated relative to 70a. Thus, a
consistent rotational interval is provided along each VIV reduction
column segment.
[0045] As will be described more fully below, the column segments
may have an axial length that is between about 1/2 times the
diameter to about 10 times the diameter. In particular, it has been
discovered that columnar segments having a length of approximately
11/2 times the diameter each rotated about 30.degree. relative to
each other will advantageously break up the vortex sheet. Vortex
shedding at one column will be out of phase with the next so that
vortex induced lifting forces are out of phase and cancel each
other. By rotating each columnar segment, a consistent rotational
angle between about 10 and 90.degree., a helical design is
approximated. Each VIV reduction columnar segment may comprise one
or a plurality of longitudinal VIV reduction discontinuities.
Generally speaking, the greater number of discontinuities per
columnar segment, the longer the columnar segment may be and still
have a desired VIV reduction effect. Various embodiments,
constructions and manufacturing of VIV reduction columns will be
discussed more fully below with reference to FIGS. 5-43.
[0046] Turning now to FIG. 3, which is a configuration of hybrid
riser, an additional application of the inventive VIV reduction
mechanism may be more fully understood in connection with a support
riser 76 having structural steel pipe inside the bundle, by which a
plurality of riser pipes 68 may be supported vertically upward from
the seabed 12 to a position close to sea surface 10, for providing
flexible riser 82 connection to floating platform 74. In this
embodiment, the VIV reduction mechanism 77 comprises of a plurality
of VIV reduction columnar segments, 78a, b, c, and d etc., each
having a VIV reduction notch 84a, b, c, and d etc. preferably a
plurality of angled notches or discontinuities 84a, b, c, and d
etc. The angle of the notch relative to the longitudinal axis of a
columnar segment 78, desirably provides a segment of a helical
notch 84. Adjacent VIV reduction columnar segments 78a and b are
each simultaneously merged and are each rotated relative to each
other at appropriate angular interval so that the notches 84a and
84b are lined end to end form a cylindrical notch comprised of a
plurality of segments 84b, c, d, e, f, g, and etc. The number of
columnar segments required to provide the VIV reduction system
along the length of riser support 76 that is exposed to currents
will depend upon the depth of the currents and the length of each
columnar segment.
[0047] In the embodiment shown in FIG. 3, additional buoyancy
polymeric foam segments 80a, b, c and etc. are also provided
secured to the cylindrical riser support structure 76 toward the
top thereof where it may be tethered through cables 88 to a
production platform 74 floating on the sea surface 10. A connection
head 90 is provided by which the risers 68 are in fluid
communication with flexible risers 82 to provide hydrocarbons to
the surface vessel.
[0048] Referring now to FIG. 4, one embodiment of a riser support
column with risers encased in a foam retaining material is
schematically depicted with a partial perspective view of one
portion of a riser support cylinder assembly having foam material
in cylindrical quadrants encasing a plurality of risers and further
providing additional buoyancy VIV reduction mechanisms clamped
around the periphery of the cylindrical foam structure.
Particularly, a metal cylinder 102 provides the main riser support
and a plurality of petroleum recovery risers 104a, 104b, 104c, 104d
are provided along with control cables 106a and 106b as well as
additional pressurizing pipes 108a, b and 108c and d as well as gas
recovery pipes 110a and 110b (110b not shown in FIG. 4). The VIV
columnar segments 70a, 70b, 70c and 70d are shown constructed of
four VIV reduction column sections, the risers, conduits and
control cables extending along the length of support cylinder 102
being are encased within four molded polymeric foam sections 120,
122, 124 and 126 making up each of the columnar segments 70a, 70b,
70c and 70d. Adjacent ones of sections 120, 122, 124 and 126, need
not be the same cross-sectional shape, although it is preferred
that respectively opposing sections, i.e., 120 and 126, and 122 and
124, be the same shape as their opposed section. These sections are
respectively "split" at junctions 146 and 148 (not shown if FIG. 4,
see FIG. 5) for petroleum recovery risers 104a, 104b, 104c and 104d
and include half-circle cutouts for these risers. Sections 122 and
126 include outwardly open cut-outs for cables 106a and 106b, and
sections 120 and 124 include inwardly open cut-outs for gas
recovery lines 110a and 110b. The construction of these sections
will be more fully understood with reference also to FIG. 5 which
is a cross-sectional view of VIV reduction riser assembly according
to FIG. 4 taken along section line 5-5. two of which 128 and
130.
[0049] Each VIV reduction segment 70a, 70b, 70c and 70d has a
discontinuity 132a, 132b, 132c and 132d in its outer surface, and a
corresponding discontinuity 132a', 132b', 132c' and 132d' on the
outer surface of its back side. As depicted in FIG. 4, each of
these discontinuities comprises a substantially radially directed
face 134 extending inward from the exterior surface 142, a distance
approximating between {fraction (1/10)}th and {fraction (3/10)}ths
the diameter thereby decreasing the wall thickness of VIV reduction
columnar half 130 as depicted at 136. A substantially flat surface
140 is formed projecting substantially at right angles to face 134
thereby providing a right triangular notch 132. Subsequent columnar
segments 70a, 70b and 70c also have a similar notches 132a, 132b
and 132c, respectively. In the embodiment depicted in FIGS. 4 and
5, two opposed ones of the four columnar segments also has a
discontinuity or a notch 132 formed in its face. These sections are
clamped using clamps 142 and 144 to securely hold the additional
buoyancy foam, into which the VIV reduction mechanism has been
formed, onto the exterior of the cylindrical riser assembly 80. At
junctions 146 and 148 (not shown in FIG. 4, see FIG. 5) between the
sections, the wall thickness of the adjacent VIV reduction column
sections is the same.
[0050] Referring to FIG. 5 that is a cross-sectional view of the
VIV reduction riser assembly of FIG. 4, it can be seen that the VIV
reduction columns according to this embodiment have substantially
concentric notches at opposite sections where the thickness of the
wall is reduced an equivalent amount D on each side and the wall
thickness progressively increases from that notch 132 toward the
opposing section, where the diameter continues to increase until
the second notch 132 on that opposing section is reached. Again,
the discontinuity wall thickness is decreased the distance D and
again the wall thickness progressively increases past the junction
148 until the subsequent notch 132 on the other side is reached.
Similar structure is provided with respect to each of the VIV
reduction columnar segments 70a, 70b, 70c and 70d, in which
successive segments are mounted sequentially adjacent to each other
except rotated a predetermined angular interval between zero and
90.degree.. It has been found that rotation of approximately
30.degree. provides good VIV reduction, thus discontinuity 132b is
offset from the prior discontinuity 132 by an angle of
approximately 30.degree.. Subsequent columnar segment 70c is
likewise formed with four sections. The foam segments of these
successive of these columnar segments are molded such that each
successive discontinuity 132 is rotated about 30.degree.. with
respect to the next. It has further been found that the length 144
of each columnar segment 170a, b, c, etc. may be desirably about
1.5 times the nominal diameter of the VIV reduction columnar
segments.
[0051] Turning now to FIG. 6, a cross-section another embodiment of
the VIV suppression device surrounding a pipe 108' is depicted
having four discontinuities or "notches" 158, 159, 160 and 161
formed in four quadrants of the VIV columnar segment. The eccentric
exterior shape retains or approximates a substantially cylindrical
columnar shape. In this embodiment, the VIV suppression device may
conveniently be molded onto the pipe, or slipped onto its end prior
to installation of the pipe.
[0052] FIG. 7 shows an embodiment similar to FIG. 6, except that
each VIV reduction columnar segment is divided into two
substantially identical pieces, to facilitate assembly. The cuts
163 and 164 can be anywhere in the segment.
[0053] FIG. 8 shows another embodiment similar to FIG. 7, except
that the discontinuities 158, 159,160 and 161 are, for example, at
or near the junctions between each quadrant. In this embodiment,
each VIV reduction columnar segment is divided into four identical
pieces which lock each other together at zig-zag split lines 166,
167, 168, 169. This embodiment permits the load on the notches to
be better distributed along the entire length of the segment.
[0054] FIG. 9 is a schematic depiction of a VIV reduction mechanism
180 formed of a plurality of VIV reduction columnar segments 181a,
b, c, d, e, f, g, h, i, j, k and l stacked in an elongated column
each having a longitudinal discontinuity 182 in the form of notches
182a, b, c, d, e, f, g, h, i, j, k and l. For clarity only one
notch on each columnar segment is shown. Each columnar segment is
rotated 30.degree. degrees relative to each other. By sequentially
rotating the columnar segments 181, the notches 182 are arranged in
a pattern that approximates a helical pattern. The rotation angle
of 30.degree. provides twelve columnar segments for one complete
helical rotation of the vertical notch positions.
[0055] FIGS. 10, 11, 12 and 13 are schematic cross-sectional views
taken at section lines at 10-10, 11-11, 12-12 and 13-13,
respectively. Each cross-sectional depiction represents 90.degree.
rotation or each third one of the columnar sections each rotated
30.degree.. In FIG. 10 an indication of a perspective view is
depicted in phantom lines in combination with the solid line
cross-sectional view to assist in visualization of the construction
of the discontinuity or notch 182a. Although the embodiment
depicted shows a cross-section of a substantially cylindrical
column segment that is slightly eccentric rather than perfectly
cylindrical, the construction may be understood in terms of a
nominal diameter D represented by numeral 184. Referring again to
FIG. 9 the height of each column 185 is conveniently in a range of
between one half times D to about five times D, to permit
offsetting of the discontinuities by the desired rotation angle,
however, the ratio is not critical to the invention. Longer
columnar segments might be used, for example, where a plurality of
notches 182 are formed in each columnar segment rather than the
single notch as depicted in FIGS. 9 through 13. The notch or
discontinuity has a substantially flat face 183 that provide a
corner along the length of 185 of the column. The face has a depth
B represented by numeral 187 into the eccentric surface of the
cylindrical column 181 a. Depth B consist of a portion C
represented by numeral 188 that accomplishes the eccentricity of
the columnar segment and the remainder which corresponds to the
reduction in the radius less than the nominal diameter D. The size
of the notch depends upon the specific conditions of use. Of
course, the rotation need not be 30 degrees, as any offset
sufficient to create any pattern of notches effective to diminish
VIV will suffice. Again with reference also to FIGS. 10, 11, 12 and
13 each of which depicts a cross-sectional view of the VIV
reduction mechanism 190 at Section lines 10-10, 11-11, 12-12, and
13-13, respectively. In the embodiment depicted in FIGS. 19 through
13 as more specifically set forth with reference to FIGS. 10 and
11, the cylindrical columnar segments 192 have a diameter D
represented by numeral 194. The longitude and the length of each
column is between one-half times D and five times D as represented
by reference rule 195. The discontinuity or notch 192 a has a flat
face 193 that is radiantly aligned with the central axis of the VIV
columnar segment 191a and has a flat surface 195 projecting at
right angles from face 193. This produces a sharp exterior corner
at 198 that facilitate initiation of the shear shedding as
discussed previously. The depth of the phase B represented by
numeral 197 may be in the range of 0.1 to 0.3 times the diameter D.
The face 195 has a width A represented by numeral 196 that may be
in the range of 0.3 to 0.8 times the nominal diameter D.
[0056] FIG. 14 depicts a side view of sequentially arranged
segments with notches formed at an angle into the outer surface of
the VIV reduction device, so that when the segments are
successively arranged, the notches form a substantially
longitudinally continuous spiral notch. Each columnar segment
rotate at 30.degree. relative to the other as with 90 degrees of
rotation. The arrangement of each third segment is depicted in
cross-sections in FIGS. 15, 16, 17 and 18.
[0057] FIG. 19 shows another embodiment in which the outline of the
columnar segment is not exactly a circle; i.e., it is somewhat
spiral-shaped. The phantom line 199 in the drawing shows a circle
but is not part of a structure. At one side of the surface extends
beyond the circular phantom line and at the other side it is inside
the circular phantom line. The notch sequential off-setting
arrangement in the longitudinal direction can be the same as
depicted in FIGS. 9 and 14; i.e., approximately 30 degrees..
[0058] FIG. 20 shows another embodiment similar to FIG. 19, except
that the columnar VIV reduction segment is divided into two
identical pieces at cut lines 163' and 164'. The notch arrangement
in the longitudinal direction can be the same as depicted in FIGS.
9 and 14; i.e., approximately 30 degrees.
[0059] FIG. 21 shows another embodiment that has a notch 158" of a
different shape; i.e., a square. The notch arrangement in the
longitudinal direction can be the same as depicted in FIGS. 9 and
14. Although only one notch 158" is depicted, four or any number
could be used, as in FIGS. 9 and 14.
[0060] FIG. 22 is another embodiment which has a cross section as
shown in FIG. 23. The triangular notches 300 cover entire
cylindrical surface and in the longitudinal direction, the notches
form spiral (helical) lines.
[0061] This embodiment uses a VIV reduction mechanism in which a
plurality of V-type notches 300 are equilateral triangles are
formed into the surface of the substantially cylindrical column.
Again the star-shaped cross-section of FIG. 23 continuously spirals
along the length of the column depicted in FIG. 23. This may be
created by a long columnar section longer than the one-half to ten
times the diameter columns that might be more appropriate with
vertically aligned notches. However for ease of manufacture and for
clamping onto cylindrical risers or cylindrical riser support
structures or the like columnar sections might still be used and
alignment will be easily accomplished because of the uniform star
shape provided by the plurality of V-shaped notches.
[0062] FIG. 24 and 25 show another embodiment where the cross
section 250 is slightly twisted, an ellipse, successive segments
being offset about 45 degrees so the long axis of the ellipse
"spirals," as shown in FIG. 25, to form a spiral (twisted)
shape.
[0063] FIGS. 26 and 27 show another embodiment where the cross
section 255 is a slightly twisted triangle with rounded corners.
Successive segments are offset about 45 degrees, the direction of
the triangle, as shown in FIG. 27, to form a spiral (twisted)
shape.
[0064] FIGS. 28 and 29 show another embodiment where the cross
section is a square with rounded corners. The angular orientation
of the square rotates, as shown in FIG. 29, to form a spiral
(twisted) shape.
[0065] FIGS. 30 and 31 show another embodiment where the cross
section is an ellipse. The angular orientation of the long axis of
the ellipse rotates as shown in FIG. 31, to form a discontinuous
stepped pattern.
[0066] FIGS. 32 and 33 show another embodiment where the cross
section is a triangle with rounded corners. The angular orientation
of the triangle rotates, as shown in FIG. 33, to form a
discontinuous stepped pattern.
[0067] FIGS. 34 and 35 show another embodiment where the cross
section is a square with rounded corners. The angular orientation
of the square rotates, as shown in FIG. 35, to form a discontinuous
stepped pattern.
[0068] While this invention has been described with reference to
illustrative embodiments, this description is not intended to be
construed in a limiting sense. Various modifications and
combinations of the illustrative embodiments, as well as other
embodiments of the invention, will be apparent to persons skilled
in the art upon reference to the description. It is, therefore,
intended that the appended claims encompass any such modifications
or embodiments. Other alterations and modifications of the
invention will likewise become apparent to those of ordinary skill
in the art upon reading the present disclosure, and it is intended
that the scope of the invention disclosed herein be limited only by
the broadest interpretation of the appended claims to which the
inventors are legally entitled.
* * * * *